The Lee Tunnel is part of the Thames Tideway Scheme and runs from Abbey Mills Pumping Station down to Beckton Sewage Treatment Works. We have worked on several parts of the scheme for several different contractors.
Initially our involvement was with Slipform international for the lining of a 40m diameter shaft. The standard Slipform equipment had to be adapted for a single-sided shaft lining with a tapering rebate. Following construction of the tunnels the equipment was adapted a second time to form a tapering wall keyed into the rebate at each side of the shaft. To enable the formwork to taper, the normal fixed joints between members, on which the rig relies for stability, have to be pinned. Alternative load paths were introduced to achieve this and a sophisticated computer analysis was made to convince the client that the rig was stable.
The outfall from the Lee Tunnel was constructed as an immersed tube installed in a dredged channel. We designed free-flow sheet pile retaining walls to support the ground adjacent to the channel but to allow the 3m diameter tubes to be floated into position.
At the end of the outfall there is an RC headwall which is below low water level which we were asked to design by the contractor. We designed the permanent works so that the formwork could be used as a bulkhead to enable the headwall to be floated into position and installed at the end of the free-flow retaining walls.